1. Field of the Invention
The present invention relates to a method of and an apparatus for, and a recording medium with a recorded computer program for, searching for an unknown specimen, i.e., identifying an unknown polymer such as a plastic or synthetic resin.
2. Description of the Prior Art
It has heretofore been known to identify an unknown polymer by producing data of the unknown polymer according to a process (Py-GC/MS) which is a combination of pyrolysis gas chromatography (Py-GC) and mass spectrometry (MS), and comparing the produced data with a library. Specifically, the data of the unknown polymer are compared with data of known polymers stored in the library to search for the data of the known polymer which agree with the data of the unknown polymer.
According to the Py-GC/MS process, a pyrolytic product of the unknown polymer is separated by a capillary column and analyzed by a mass spectrometer, which detects the intensities of molecular ions and fragment ions that are generated when an electron beam is applied to separated molecules. In the mass spectrometer, each time an electron beam is applied to the separated molecules for a predetermined period of time, e.g., 0.008 minute, one set of ion intensities is obtained as detected data. The detected ion intensities of each set are added to calculate a total ion intensity, and the total ion intensities of the sets are plotted successively at detected times, producing a pyrolysis gas chromatogram (pyrogram) as shown in FIG. 5 of the accompanying drawings.
The pyrogram itself may be compared with the pyrogram of a known polymer specimen to identify the unknown polymer. According to the Py-GC/MS process, however, it is customary to generate mass spectra of peaks of the pyrogram and search a library of mass spectra of known polymers based on the generated mass spectra.
For example, when the ion intensities of total ion intensities that make up portions corresponding to the maximum points of the peaks, as representative values of the total ion intensities, are arranged in the order of masses of the molecular ions and the fragment ions, there is obtained a mass spectrum having a pattern inherent in compounds corresponding to the peaks. The mass spectrum is compared with a library of mass spectra produced with respect to standard specimens to search for a mass spectrum in the library that agrees with the mass spectrum for thereby identifying the unknown polymer.
The above conventional library searching process will be described below with respect to an example in which a pyrogram shown in FIG. 5 of the accompanying drawings is plotted as the pyrogram of an unknown polymer. According to the conventional library searching process, the total ion intensity that makes up a portion corresponding to the maximum point of a peak D is divided into individual ion intensities. Then, when the individual ion intensities are arranged in the order of masses of molecular ions and fragment ions, a mass spectrum shown in FIG. 6 of the accompanying drawings is obtained. The mass spectrum shown in FIG. 6 is then compared with a library of mass spectra produced with respect to standard specimens to search for the mass spectrum of a known polymer in the library that agrees with a high probability percentage with the mass spectrum for thereby identifying the unknown polymer.
When a commercial library (WILEY) is searched for the mass spectrum shown in FIG. 6, a result is obtained as shown in FIGS. 10(a) through 10(d). FIGS. 10(a) and 10(b) show mass spectra of 4,4xe2x80x2-(1-methylethylidene) bisphenol. FIG. 10(c) shows a mass spectrum of p-phenyl-carbanilic acid, and FIG. 10(d) shows a mass spectrum of [1,1xe2x80x2-biphenyl]-3-yl-carbamic acid. The probability that a pyrolytic product corresponding to the mass spectrum shown in FIG. 6 is the same as the compounds shown in FIGS. 10(a) through 10(d) is 97%, 83%, 46%, and 43%, respectively.
According to the conventional library searching process, each of the peaks of the pyrogram shown in FIG. 5 is processed in the same manner as with the peak D to identify compounds corresponding to the peaks.
However, the conventional library searching process is time-consuming because the library needs to be searched for each one of the pyrolytic products corresponding to the peaks. Even if each one of the compounds corresponding to the peaks is identified, it requires a high level of expert knowledge, experience, and skill to determine the original polymer from the compounds thus identified.
Another process of searching for an unknown polymer uses a mass spectrum measured by directly pyrolyzing the polymer in a mass spectrometer. According to this process, one mass spectrum is obtained with respect to one polymer. Therefore, it is not necessary to search a library for individual mass spectra corresponding to peaks, and it is possible to determine an original polymer immediately from the search result.
The above process requires that a specimen introduced into the mass spectrometer be in a minute quantity of several xcexcg or less. However, it is practically difficult to weigh such a minute quantity of specimen. Polymers that are dissolvable into solvents can be weighed in minute quantities of several xcexcg by adjusting the concentration of the solution. However, such a weighing scheme is not applicable to crosslinked polymers that are not dissolvable into solvents.
Another problem is that when a specimen of several hundred xcexcg that can be weighed is pyrolyzed, the ionization chamber in the mass spectrometer is contaminated and becomes useless in a short period of time.
It is therefore an object of the present invention to provide a method and apparatus, along with a recording medium with a recorded computer program, for easily identifying an unknown polymer such as a plastic or synthetic resin.
To achieve the above object, there is provided in accordance with the present invention a method of searching for an unknown polymer, comprising the steps of separating a mixture of pyrolytic products obtained by pyrolyzing an unknown polymer into the pyrolytic products, detecting ion intensities of molecule ions and fragment ions produced by ionizing the separated pyrolytic products in cycles each for a predetermined period of time, adding sets of the ion intensities of the molecule ions and the fragment ions detected in the cycles into total ion intensities, interconnecting maximum levels of the total ion intensities in a histogram composed of the total ion intensities arranged in the order of detection times thereof for thereby plotting a chromatogram, combining, for each ion of the same mass, the ion intensities contained in all the total ion intensities making up peaks in the chromatogram into combined data of the ion intensities of the molecular ions and the fragment ions, generating a combined mass spectrum of the unknown polymer which is composed of the combined data arranged in the order of masses of the molecular ions and the fragment ions, and comparing the generated combined mass spectrum of the unknown polymer with a library of combined mass spectra of a plurality of known polymers which are generated in the same manner as with the combined mass spectrum of the unknown polymer, for thereby searching for the combined mass spectrum of one of the known polymers which agrees with a high probability percentage with the combined mass spectrum of the unknown polymer.
In the above searching method, a mixture of pyrolytic products obtained by pyrolyzing an unknown polymer is separated into the pyrolytic products. The pyrolytic products are ionized, and ion intensities of generated molecule ions and fragment ions are detected. Then, total ion intensities as totals of the ion intensities detected in cycles each for a predetermined period of time are calculated, and arranged in the order of detection times thereof thereby to generate a chromatogram of the unknown polymer.
The peaks in the chromatogram which correspond to the pyrolytic products and which are formed at respective times required to separate them (retention times) represent clusters of the total ion intensities. Each of the total ion intensities may contain molecule ions or fragment ions of the same mass. Therefore, the ion intensities included in all the total ion intensities making up the peaks are combined, for each ion of the same mass, to produce combined data.
The combined data are arranged in the order of the masses of the molecule ions and the fragment ions thereby to obtain a combined mass spectrum of the unknown polymer. The combined mass spectrum corresponds to a mass spectrum which is produced by directly analyzing the mixture of pyrolytic products of the unknown polymer with a mass spectrometer, rather than separating the mixture, and arranging the ion intensities of detected molecule ions and fragment ions in the order of their masses. Consequently, the combined mass spectrum constitutes a single mass spectrum capable of representing data relative to the unknown polymer.
The combined mass spectrum of the unknown polymer is then compared with the library. The library is composed of the combined mass spectra of a plurality of known polymers which are generated in the same manner as with the combined mass spectrum of the unknown polymer. The library contains combined mass spectra generated with respect to peaks in certain ranges in the chromatograms of the known polymers, as data libraries. The peaks in the certain ranges in the chromatograms may be all or some of the peaks contained in the chromatograms.
The comparison between the combined mass spectrum of the unknown polymer with the library searches for the combined mass spectrum of one of the known polymers which agrees with a high probability percentage with the combined mass spectrum of the unknown polymer. The unknown polymer is thus identified as the known polymer whose combined mass spectrum agrees with a high probability percentage with the combined mass spectrum of the unknown polymer.
An apparatus for searching for an unknown polymer according to an aspect of the present invention comprises pyrolyzing means for pyrolyzing a specimen to generate a mixture of pyrolytic products, a separating column for separating said mixture into the pyrolytic products, a mass spectrometer for detecting ion intensities of molecule ions and fragment ions of the separated pyrolytic products, chromatogram generating means for generating a chromatogram from the ion intensities of the molecule ions and the fragment ions, combining means for generating a combined mass spectrum of the unknown polymer, library memory means for generating and storing a library of combined mass spectra of a plurality of known polymers, searching means for searching the library and outputting a search result. The method for searching for an unknown polymer according the present invention can effectively be carried out by the above apparatus.
An apparatus for searching for an unknown polymer according to another aspect of the present invention comprises library memory means for storing a library of combined mass spectra of a plurality of known polymers, detected data memory means for storing ion intensities of molecule ions and fragment ions of an unknown polymer as detected data, chromatogram generating means for generating a chromatogram based on total ion intensities obtained from the detected data, combining means for generating a combined mass spectrum of the unknown polymer, and searching means for comparing the library with the combined mass spectrum of the unknown polymer to search for the combined mass spectrum of the unknown polymer. The above apparatus may comprise a personal computer.
If the apparatus for searching for an unknown polymer comprises a personal computer, then a recording medium storing an application program executable by the personal computer may be employed. The recording medium records therein a process comprising the steps of storing ion intensities of molecule ions and fragment ions of an unknown polymer as detected data, generating a chromatogram based on total ion intensities obtained from the detected data, generating a combined mass spectrum of the unknown polymer, and comparing the library with the combined mass spectrum of the unknown polymer to search for the combined mass spectrum of a known polymer which agrees with a high probability percentage with the combined mass spectrum of the unknown polymer. A recording medium which stores the library described above may also be employed. Each of these recording mediums may comprise a magnetic recording medium such as a floppy disk or the like, or an optical disk such as a CD-ROM or the like.
The above and other objects, features, and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate a preferred embodiment of the present invention by way of example.